The traditional measurement of renal blood flow in ml/min involves the collection of multiple blood and urine samples after an infusion of a drug called para-aminohippurate (PAH) is given. In this study, the investigators determine the ability of MRI to estimate renal blood flow. The primary hypothesis is that renal blood flow as measured by PAH clearance is similar to renal blood flow as estimated by MRI. The measurements by PAH clearance and MRI are both performed on the same day.